For over three decades, the electron microscope has been an integral part of the research program of the Epidemiology Section regarding the etiologic role, natural history and physical characteristics of various important enteric viruses. Past highlights of these investigations have included inquiry into the etiology of diseases of unknown cause involving studies of fastidious agents of disease that defied cultivation in any cell culture system. The electron microscope played a major role (i) in the discovery of the 27nm Norwalk virus (now known as a member of the norovirus genus in the calicivirus family) and the establishment of its etiologic role in human diarrheal disease, and (ii) in collaboration with the Hepatitis Viruses Section (HVS), in the discovery of the 27nm hepatitis A virus particle and establishment of its etiologic role in infectious hepatitis and in addition in studies of the hepatitis E virus with the HVS . It also was used as an important epidemiologic tool for the detection of rotaviruses in stools of infants and young children hospitalized with diarrhea and helped in establishing the importance of these agents in severe diarrhea of infants and young children. Although intensive efforts have been made, the Norwalk and related human enteric noroviruses have yet to be cultivated in any tissue culture system. This, along with the visualization of virus-like particles (VLPs) of certain members of this group of agents, have contributed to the continuing important role of the electron microscope in the research program of the LID. The electron microscope has continued to be an important primary and adjunctive tool in various capacities. Since last year's annual report, studies have included a variety of areas: (1) visualization of full walrus vesivirus VLPs in a purified preparation following dialysis;(2) visualization of Australian calicivirus VLPs from 2001, 2007 and 2008 (collaboration with Ms. Mahers and Dr. Kirkwood, Pediatrics, Royal Childrens Hospital, Australia);(3) visualization of chimeric VLPs of Norwalk virus shell and JHH3 P (collaboration with JHU);(4) attempt to determine if ascorbate has any direct effect on rotavirus SA-11 morphologically (collaboration with Dr. Levine of NIDDK);(5) attempts to visualize hepatitis E virus by direct EM, by IEM without centrifugation and IEM with centrifugation (collaboration with Dr. Emerson);(6) attempt to determine if hepatitis E virus genotype is genotype specific by IEM;(7) visualization of Norwalk virus- shell murine norovirus P domain chimeric capsid proteins expressed in baculovirus system;( 8) attempt to visualize morphologic appearance of influenza virus H1N1 by direct EM (collaboration with Dr. Lackdawala and Ms. Lamirande);(9) visualization of various murine norovirus preparations in CsCl;(10) visualization of presence and integrity of VLP preparation of norovirus JHH3 being prepared for collaborative cryo- EM study at RML.